a) Field of the Invention
An invention directed to an electrophoretic display comprising isolated cells filled with charged pigment particles dispersed in a dielectric solvent. The filled cells are individually sealed with a polymeric sealing layer.
b) Description of Related Art
The electrophoretic display is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by using spacers. One of the electrodes is usually transparent. A suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment or the color of the solvent can be seen according to the polarity of the voltage difference.
In order to prevent undesired movement of the particles, such as sedimentation, partitions between the two electrodes were proposed for dividing the space into smaller cells. However, in the case of partition-type electrophoretic displays, some difficulties were encountered in the formation of the partitions and the process of enclosing the suspension. Furthermore, it was also difficult to keep different colors of suspensions separate from each other in the partition-type electrophoretic display.
Subsequently, attempts were made to enclose the suspension in microcapsules. U.S. Pat. Nos. 5,961,804 and 5,930,026 describe microencapsulated electrophoretic displays. The reference display has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a suspension of charged pigment particles that visually contrast with the dielectric solvent. The microcapsules can be formed by interfacial polymerization, in-situ polymerization or other known methods such as physical processes, in-liquid curing or simple/complex coacervation. The microcapsules, after their formation, may be injected into a cell housing two spaced-apart electrodes, or xe2x80x9cprintedxe2x80x9d into or coated on a transparent conductor film. The microcapsules may also be immobilized within a transparent matrix or binder that is itself sandwiched between the two electrodes.
The electrophoretic displays prepared by these prior art processes, in particular the microencapsulation process as disclosed in U.S. Pat. Nos. 5,930,026, 5,961,804, and 6,017,584, have many shortcomings. For example, the electrophoretic display manufactured by the microencapsulation process suffers from sensitivity to environmental changes (in particular sensitivity to moisture and temperature) due to the wall chemistry of the microcapsules. Secondly the electrophoretic display based on the microcapsules has poor scratch resistance due to the thin wall and large particle size of the microcapsules. To improve the handleability of the display, microcapsules are embedded in a large quantity of polymer matrix which results in a slow response time due to the large distance between the two electrodes and a low contrast ratio due to the low payload of pigment particles. It is also difficult to increase the surface charge density on the pigment particles because charge-controlling agents tend to diffuse to the water/oil interface during the microencapsulation process. The low charge density or zeta potential of the pigment particles in the microcapsules also results in a slow response rate. Furthermore, because of the large particle size and broad size distribution of the microcapsules, the prior art electrophoretic display of this type has poor resolution and addressability for color applications.
The first aspect of the present invention is directed to an electrophoretic display comprising cells of well-defined shape, size and aspect ratio. The cells are filled with an electrophoretic fluid comprising charged particles dispersed in a dielectric solvent and are individually sealed with a polymeric sealing layer. The polymeric sealing is preferably formed from a composition comprising a thermoset or thermoplastic precursor. In one embodiment of the invention, the cells are partially filled with electrophoretic fluid above which the sealing layer forms a contiguous film and is in intimate contact with both the fluid and the peripheral of the cell walls that are not covered by the fluid. In one of the preferred embodiments of the invention, the sealing layer further extends over the top surface of the cell side walls.
In another preferred embodiment of the invention, the top surface of the cell walls is at least 0.01 micrometer (xcexc) above the top surface of electrophoretic fluid. More preferably, the top surface of the cell walls is about 0.02xcexc to 15xcexc above the top surface of electrophoretic fluid. Most preferably, the top surface of the cell walls is about 0.1xcexc to 4xcexc above the top surface of electrophoretic fluid.
In another preferred embodiment of the invention, the top surface of the polymeric sealing layer is at least 0.01xcexc above the top surface of the cell walls to improve the adhesion between the sealing layer and the cells. More preferably, the top surface of the polymeric sealing layer is about 0.01xcexc to 50xcexc above the top surface of the cell walls. Even more preferably, the top surface of the polymeric sealing layer is about 0.5xcexc to 8xcexc above the top surface of the cell walls. The total thickness of the sealing layer is about 0.1xcexc to 50xcexc, preferably about 0.5 to 15xcexc, more preferably 1xcexc to 8xcexc. Most preferably, the sealing layer forms a contiguous film above the cell walls and the electrophoretic fluid.
Another aspect of the invention relates to a novel process for the manufacture of such an electrophoretic display.
A further aspect of the invention relates to the preparation of cells of well-defined shape, size and aspect ratio. The cells enclose a suspension of charged pigment particles dispersed in a dielectric solvent and are formed from microcups prepared according to the present invention. Briefly, the process for the preparation of the microcups involves embossing a thermoplastic or thermoset precursor layer coated on a conductor film with a pre-patterned male mold, followed by releasing the mold during or after the thermoplastic or thermoset precursor layer is hardened by radiation, cooling, solvent evaporation, or other means. Alternatively, the microcups may be formed from imagewise exposure of the conductor film coated with a radiation curable layer followed by removing the unexposed areas after the exposed areas have become hardened.
Solvent-resistant, thermomechanically stable microcups having a wide range of size, shape, and opening ratio can be prepared by either one of the aforesaid methods. The microcups are then filled with a suspension of charged pigment particles in a dielectric solvent, and sealed.
Yet another aspect of the present invention relates to the sealing of the microcups filled with the electrophoretic fluid containing a dispersion of charged pigment particles in a dielectric fluid. Sealing can be accomplished by a variety of ways. One of the preferred embodiments, is accomplished by dispersing a sealing composition comprising a thermoplastic, thermoset, or their precursors in the electrophoretic fluid before the filling step. The sealing composition is immiscible with the dielectric solvent and has a specific gravity lower than that of the solvent and the pigment particles. After filling, the sealing composition phase separates from the electrophoretic fluid and forms a supernatant layer at the top of the fluid. The sealing of the microcups is then conveniently accomplished by hardening the sealing layer by solvent evaporation, interfacial reaction, moisture, heat, or radiation. UV radiation is the preferred method to harden the sealing layer, although a combination of two or more curing mechanisms as described above may be used to increase the throughput of sealing.
In another preferred embodiment, the sealing can be accomplished by overcoating the electrophoretic fluid with a sealing composition comprising a thermoplastic, thermoset, or their precursors. The sealing is then accomplished by hardening the precursor by solvent evaporation, interfacial reaction, moisture, heat, radiation, or a combination of curing mechanisms. These sealing processes are especially unique features of the present invention. Additives such as surfactants, leveling agents, fillers, binders, viscosity modifiers (thinning agents or thickeners), co-solvents, and antioxidants may be added to the sealing composition to improve the display performance. Dyes or pigments may also be added in the sealing layer particularly if the display is viewed from the opposite side.
Yet another aspect of the present invention relates to a multiple step process for the manufacture of a monochrome electrophoretic display. The processing steps include preparation of the microcups by any one of the methods described above, sealing of the microcups, and finally laminating the sealed array of microcups with a second conductor film with an adhesive. This multiple-step process can be carried out roll to roll continuously.
Yet another aspect of the present invention relates to a process for the manufacture of a full color electrophoretic display by laminating or coating the preformed microcups with a layer of positively working photoresist, selectively opening a certain number of the microcups by imagewise exposing the positive photoresist, followed by developing the resist, filling the opened cups with a colored electrophoretic fluid, and sealing the filled microcups by one of the sealing processes of this invention. These steps may be repeated to create sealed microcups filled with electrophoretic fluids of different colors.
These multiple-step processes as disclosed may be carried out roll-to-roll on a web continuously or semi-continuously. The microcup structure in fact enables a format flexible and efficient roll-to-roll continuous manufacturing. These processes are very cost effective as compared to typical display manufacturing processes.
One advantage of the electrophoretic display (EPD) of this invention is that the microcup wall is in fact a built-in spacer to keep the top and bottom substrates apart at a fixed distance. The mechanical properties and structural integrity of this type of display is significantly better than any prior art displays including those manufactured by using spacer particles. In addition, displays involving microcups have desirable mechanical properties including reliable display performance when the display is bent, rolled, or under compression pressure from, for example, a touch screen application. The use of the microcup technology also eliminates the need of an edge seal adhesive to predefine the size of the display panel and confine the display fluid inside a predefined area. The display fluid within a conventional display prepared by the edge sealing adhesive method will leak out completely if the display is cut in any way, or if a hole is drilled through the display. The damaged display will be no longer functional. In contrast, the display fluid within the display prepared by the microcup technology is enclosed and isolated in each cell. The microcup display may be cut into almost any dimensions without the risk of damaging the display performance due to the loss of display fluid in the active areas. In other words, the microcup structure enables a format flexible display manufacturing process, wherein the process produces a continuous output of displays in a large sheet format which can be cut into any desired sizes.
The isolated microcup or cell structure is particularly important when cells are filled with fluids of different specific properties such as colors and switching rates. Without the microcup structure, it will be very difficult to prevent the fluids in adjacent areas from intermixing or being subject to cross-talk during operation. As a result, the bistable reflective display of this invention also has excellent color addressability and switching performances.
The electrophoretic display prepared according to the present invention is not sensitive to environment, particularly humidity and temperature. The display is thin, flexible, durable, easy-to-handle, and format-flexible. The drawbacks of electrophoretic displays prepared by the prior art processes are therefore all eliminated.